Cancer Screening Tests

What MDs don’t know about cancer screening

Most primary care physicians are keen on cancer screening. In fact, sending symptom-free patients for regular tests is central to their practice. Yet an understanding of cancer screening statistics is critical to informed decision-making, whether you’re the doctor sending people for tests or a patient just following orders. A new survey of U.S. primary care physicians shows the majority accept misleading statistics as proof that screening works.

Four hundred and twelve physicians took the online survey, which was designed by an American and German research team with a history of trying to improve understanding of health statistics by health professionals as well as the general public. “Most physicians incorrectly equated improved survival and early detection as evidence of lives saved,” concluded the researchers led by Odette Wegwarth, PhD, Max Planck Institute for Human Development, Berlin, Germany. “Few correctly recognized that only reduced mortality in a randomized trial constitutes evidence of the benefit of screening.” The survey results were published this week in Annals of Internal Medicine.

The survey presented physicians with two ways of expressing the effect of a hypothetical screening test which was described as improved 5-year survival and increased early detection in one scenario and as decreased cancer mortality and increased incidence in the other. Though the type of cancer was not identified, the hypothetical test scenarios were based on real-life data from the European prostate-cancer screening randomized trial. And the 5-year survival statistics and the percentage of stage I prostate cancers came from the U.S. database of cancer statistics collected in 1975. To be safe, that year was chosen because it predates the introduction of any organized screening program for prostate cancer.

The physicians were more impressed with what the survey authors called “irrelevant evidence,” for example, a test with a large 5-year survival rate. Here’s why this is irrelevant: The older we get, the more cancers we have in our bodies; many will never become life-threatening. Prostate cancer, for example, is in the overwhelming majority of cases a slow-growing or non-progressive cancer.

Therefore, prostate cancer’s 5-year survival rate will look like a clear justification for early cancer detection because most men will die of something else. They could die six years after diagnosis and still be counted as a “survivor”. Furthermore, screening often moves up the time of diagnosis (and treatment) without moving back the time of death. (By the way, we can thank the American Cancer Society for its long-time use of this extremely misleading measurement of a cancer screening test’s benefit. In the not-so-distant past, the ACS actually used the word ‘cure’ interchangeably with 5-year survival, thus making generations of cancer patients think that making it to five years meant something.)

Now for the other worrisome finding: The surveyed physicians were less impressed with a test described as having “reduced mortality”. And they were more impressed with a test that finds lots of cancer. But screening for cancer will always increase the number of cancer cases diagnosed, compared with the number of cancers found in people who seek medical attention only after symptoms appear. That’s because screening detects many more cancers that do not progress, which falsely inflates the apparent benefit of a screening tests (a phenomenon that the survey authors describe as overdiagnosis). This is why careful researchers will—after many years of follow-up—-compare the overall death rate of both the screened and unscreened groups. It is the only way to sort out the people who actually achieved a life-saving benefit from those who were treated unnecessarily for a cancer they didn’t need to know about.

This comparison is also a way for researchers to determine screening’s “cost” in terms of harms. Here’s what the European prostate cancer screening trial found: For every one prostate cancer death avoided in the PSA screened men, 48 men suffered severe complications from unnecessary treatment of a non-progressive cancer.